This invention relates to electrophoretic display devices in general and more particularly to an apparatus and method for pulsing the electrodes of such a display to enable enhanced speed operation of the display.
As one will ascertain, the electrophoretic effect as employed in display devices is known in the prior art. Basically, the electrophoretic effect operates on the principle that certain particles will become electrically charged and because of being electrically charged these particles can migrate from a like charged surface to an opposite charged surface. Hence, particles which become positively charged will migrate towards a negative surface or terminal or vice versa. As indicated, this effect is well known and display devices have been fabricated utilizing this effect.
For typical examples, reference is made to U.S. Pat. No. 4,732,830 issued on Mar. 22, 1988 and entitled "Electrophoretic Display Panels and Associated Methods" to Frank J. Disanto and Denis A. Krusos, the inventors herein, and assigned to Copytele, Inc., the assignee herein. Reference is also made to U.S. Pat. No. 4,655,897 issued on Apr. 7, 1987 to Frank J. Disanto and Denis A. Krusos, and entitled "Electrophoretic Display Panels and Associated Methods" and also assigned to Copytele, Inc. Reference is also made to U.S. Pat. No. 4,746,917 issued on May 24, 1988 to Frank J. Disanto and Denis A. Krusos, and entitled "Method and Apparatus for Operating an Electrophoretic Display Between a Display and Non-Display Mode".
The above patents give detailed descriptions of the fabrication of such displays as well as the biasing and operation of such displays to enable the electrophoretic effect to be utilized in the production of typical display panels.
In any event, if reference is made to the above noted patents, one will see that such cells or electrophoretic displays essentially contain an anode, a cathode and a grid electrode which grid electrode further controls the transportation of charged particles. In operation, the charged particles are transferred and forced against one electrode, as the anode or cathode under the influence of an applied electric field, so that the viewer may view the color of the pigment which forms a desired display pattern. In this manner the grid electrode is employed to enable control of the migration of such particles. It is also indicated that when the polarity of the field is reversed, the pigment particles are transported and packed on the opposite electrode. This is indicative, for example, of an erasing mode.
As will be further explained, the normal voltages on a typical electrophoretic panel enable the following conditions of operation. The panel can be operated in an Erase Mode where the anode electrode is negative with respect to the cathode electrode which is positive. In this mode the grid electrodes are at a low potential which is equivalent for example to a binary 0. In a Hold Mode the anode is positive, the cathodes are positive and the grid electrodes are essentially at zero voltage or at binary 0 level. As one can understand, the cathode operates between zero and positive voltages while the grid operates between low ("0") and high voltages ("1").
As indicated above, a low condition will be indicated by a binary 0 and a high condition is indicated by a binary 1. In any event, during a Write Mode the anode is positive, the cathodes that are being written into are at zero potential and the grids, which are the writing grids, are at a positive or high potential as a binary 1. During this mode all non-writing cathodes are positive and non-writing grids are at low potential or more negative than the cathode.
In any event, as the prior art was aware of, in order to write at reasonable speeds the grid, during the writing mode, should be held at a positive potential or a high potential which is designated as a binary 1. As one will further understand, by making the grid potential positive one also operates to decrease the background brightness and causes some overwriting in areas where a grid set to 1 intersects a positive cathode line. This will be further explained in conjunction with the specification. In any event, as one will ascertain, the display is formulated by a means of intersecting parallel lines which are insulated from each other. These lines form an XY matrix or an XY array and consist of grid lines and cathode lines arranged in a matrix. Hence, to access any particular point in a matrix, one must have an X and a Y address. The X and Y address is indicated by one grid line and one cathode line which intersect to form a pixel point or area and which point or area is written by causing pigment particles to migrate out of that pixel area on said display.
At the intersection of the X and Y addresses in the matrix, one will thereby provide a writing condition. In any event, as indicated above, when the grid potential becomes positive there is a decrease in the background brightness of the display due to the fact that the potential between the grid and cathode has changed for non-writing lines.
In addition, and of greater consequence, a dark line will appear at the leading edge of the picture being written (corresponding to the cathode at zero potential). The black line is indicative of the fact that all the pigment has left the cathode in the pixels being written. This is desirable, however, it is also noted that when the potential of the cathode being written into is made positive, some of the pigment returns to the cathode resulting in incomplete writing and poor contrast. It is believed that this effect is probably due to the fact that the negatively charged pigment, which has only gone a short distance beyond the grid, is attracted back to the cathode by the combined positive grid and cathode fields. These factors substantially decrease the writing speed of such a display and provide a lack of contrast and so on, as described above.
It is the object of the present invention to apply selectable pulses to the grid and cathode electrodes during a write mode whereby the pulses supplied will serve to maintain the grid to cathode potential of non-writing electrodes at a fixed value which is indicative of a good contrast level. At the same time, a writing pulse applied to the grid remains for a given duration while the cathode to be written is held low for a longer duration. In this manner, one will have a positive grid potential at the start of writing a given pixel and a negative grid (which will repel the pigment that has traveled to the anode side of the grid) when writing of that pixel (cathode from zero to positive potential) is complete. In this manner, as will be explained, the effective speed of operation of the display is dramatically increased and results in a speed increase of approximately 6:1 over a conventional display operated according to the teachings of the prior art.
It is further indicated that the apparatus and method to be provided also pulses all cathodes which are not being written by a pulse of the same nature as the writing grid pulse. In this manner, the potential between the non-writing cathode lines and the grid lines remains constant and hence the above-noted problems are avoided.